I do not think that you have posted enough information to confirm if the truck is a 6v or 12v 1955 truck - the 1st series 1955 were 6v and the 2nd series 1955 were 12v, and anyone since 1955 might have done whatever with the electrical parts.
Does the starter or generator have a tag on it? I think that black tags indicate 6v and red tags indicate 12v. If there are tags but the color is gone, post the codes on the pages.
What are the block and head casting codes? (this will not really tell you what electrical voltage the previous owner had but it is a little more information).
Connect a good 6v battery to the starter with GOOD 6v battery leads (2 gauge cables or thicker) and see if it spins the engine. If it does not, try a 12v battery.
Do you ultimately want a 6v or 12v system?
